A gas metering arrangement of the kind referred to above is disclosed in U.S. Pat. No. 4,651,730 which is incorporated herein by reference.
With this known gas metering arrangement, small quantities of a gas at low flow velocity as well as large quantities at higher flow velocity can be metered to a consumer from a gas source. When small quantities are metered, a measuring chamber is filled via a change-over valve until a predeterminable pressure difference with reference to the pressure in a comparison-pressure chamber is reached. Thereafter, the change-over valve switches and allows the gas to flow out from the measuring chamber via a check valve which is now open to a consumer. When metering larger quantities at correspondingly higher flow velocity, an expansion chamber parallel to the measuring chamber is connected into the line supplying the expansion chamber via an expansion valve and the expansion chamber is filled. The change-over valve as well as the check valve again switch over after the pressure difference is reached so that now the measuring chamber as well as the expansion chamber can direct their respective metered gas quantities to the consumer.
With respect to the metering arrangement discussed above, it has been shown to be disadvantageous that emptying the measuring chamber and the expansion chamber to the consumer must occur via three valves when utilizing the connected expansion chamber. Dynamic pressure differences occur because of the high peak gas flows which develop because of the gas-flow resistances of the valves. These pressure differences reduce the accuracy of the metering and limit the maximum metering capacity for a predetermined static pressure condition.
In addition, the change-over valve ahead of the measuring chamber is always switched with the cycle frequency when metering smaller quantities as well as when metering larger quantities with the expansion chamber being switched in. This imposes extreme requirements with reference to the following: lower pass-through resistances, shorter switching times and thereby possibly higher cycle frequencies as well as a longer service life. The change-over valve must at the same time provide corresponding flow characteristics for low metering quantities at low flow speeds as well as large metering quantities at higher flow speeds. These requirements are difficult to realize in a single component.